1. Field of the Invention
The invention relates to an image forming apparatus for use in a copying machine, a printer, or facsimile apparatus or the like adopting, for example, an electrophotographic process or an electrostatic recording process.
2. Description of the Related Art
Heretofore, in an image forming apparatus such as a copying machine using an electrophotographic process, an electrostatic latent image formed on an image bearing member such as a photosensitive drum has been visualized by a developer being caused to adhere thereto. As this developer, there is a magnetic mono-component developer including a magnetic toner, a nonmagnetic mono-component developer including a nonmagnetic toner, a dual-component developer including a nonmagnetic toner and a magnetic carrier, or the like, and one of these is suitably used.
Of conventional developing devices in which such developers are used, an example of a developing device using the dual-component developer including the nonmagnetic toner and the magnetic carrier is shown in FIG. 8 of the accompanying drawings.
In the developing device 1 using the dual-component developer as shown in this example, there is often adopted the construction of a single sleeve developing device of a construction containing the dual-component developer in a developer container 2, and provided with a developer carrying member 8 carrying the developer thereon and conveying the developer to develop an electrostatic latent image on a photosensitive drum 10, i.e., a developing sleeve 8a and a magnet roll 8b provided therein, and further provided in the developer container 2 with conveying screws 5 and 6 which are conveying means for conveying the developer to the developing sleeve 8a while agitating the developer.
In such a developing device, however, there occurs in some cases a blank image which is one of image defects by an edge enhancement.
The mechanism of this blank image occurring will hereinafter be described with reference to FIG. 9 of the accompanying drawings. This is an example adopting a reversal developing process.
Usually, the blank image occurs when an image including a highlight image α is formed in an electrostatic latent image formed on the photosensitive drum 10, near the boundary between the highlight image α formed downstream with respect to the moving direction β of the surface of the photosensitive drum 10 and a solid image β formed upstream, i.e., between the trailing edge of the highlight image α and the leading edge of the solid image β.
FIG. 9 shows the shapes of an equipotential surface C and an electric flux line H when there is the highlight portion a on the photosensitive drum 10 and there is the solid portion β behind it and the boundary portion γ between the highlight portion a and the solid portion β is opposed to the developing sleeve 8a. It will be seen from this figure that near the boundary portion γ, the electric flux line H is greatly attracted toward the solid portion β.
Accordingly, in the conventional single-sleeve developing process, in a construction wherein the developing sleeve 8a is rotated in a forward direction D with the photosensitive drum 10, the toner in the supplied developer cannot be supplied to the trailing edge of the highlight portion α, but is used to develop along the electric flux line H toward the solid portion β and therefore, it is considered that there is a case where a blank area A occurs in the trailing edge portion of the highlight portion α.
So, in order to prevent the blank image, as shown in FIG. 10 of the accompanying drawings, there has been proposed a twin-sleeve developing process provided in a developer container 2 with two developer carrying members 8 and 9 upstream and downstream with respect to the rotation direction of a photosensitive drum 10, i.e., having developing sleeves 8a and 9a provided with magnet rolls 8b and 9b therein, and using a dual-component developer for visualizing the same electrostatic latent image on the photosensitive drum 10 by a first developing step executed by the upstream developing sleeve 8a, and a second developing step executed by the use of the downstream developing sleeve 9a (see, for example, Japanese Patent Application Laid-open No. 2003-323052).
This twin-sleeve developing process is a developing process which reduces the electric potential difference between the highlight portion a and the solid portion β by the above-described first developing step, and which reliably effects development at the trailing edge of the highlight portion α to thereby make it difficult for a blank image to occur.
Further, when the inventor carried out an investigation about an image characteristic, it was found that the dignity of image is determined chiefly by the second developing step.
Specifically, coarseness used in a measure of the quality of image and scavenging occurring in the trailing edge portion of an image are substantially determined by a developing characteristic at the second developing step. Particularly it has been ascertained that coarseness and scavenging occur due largely to such a factor as the mechanical frictional contact between the magnetic carrier in the dual-component developer and the photosensitive drum.
This will hereinafter be described in detail.
The first developing step is a step of developing an electrostatic latent image to thereby eliminate the electric potential difference between the highlight portion and the solid portion. Accordingly, it is desirable that developing efficiency be high. This can be accomplished by lengthening the effective developing time, i.e., setting the circumferential length of a developing nip to a great value. Specifically, a magnetic brush is caused to stay still after the termination of development to thereby earn the developing time, whereby the developing efficiency can be improved.
However, if the developer is caused to stay, as described above, the first developing step will be terminated with the deterioration of the image caused by a counter charge having occurred to the magnetic carrier.
When thereafter, shift is made to the second developing step, the electric potential difference between the highlight portion and the solid portion is reduced and therefore, at the second developing step, in the toner image already formed by the first developing step, the re-disposition of the toner is effected or if the toner does not adhere to that portion of the electrostatic latent image to which the toner should adhere (for example, a portion in which a blank image has occurred), the toner is supplied thereto to thereby achieve a higher quality of image.
However, the adoption of a developing method using a magnetic brush at the second developing step has led to the problem that the toner image already formed by the first developing step is disturbed more than necessary by the magnetic brush.
On the other hand, as a developing method not using a magnetic carrier, study has been made about a nonmagnetic toner mono-component developing method, as shown in FIG. 11 of the accompanying drawings (see, for example, Japanese Patent Application Laid-open No. 2004-184988).
This developing device has a developing roller 8 as a developer carrying member contacting with a photosensitive drum 10, a supplying and collecting roller 80 for supplying and collecting a toner to the developing roller 8, and an elastic blade 81 abutting against the developing roller 8. The elastic blade 81 regulates the layer thickness of the toner supplied to the developing roller 8 and also, gives predetermined triboelectrification charges to the toner.
The developing device of such a construction does not use a magnetic carrier and therefore has the advantage that there is no noise component based on the magnetic carrier and thus, a higher quality of image can be achieved easily.
However, the charge giving (hereinafter referred to as the “tribo”) to the nonmagnetic toner must be effected by only an extraneous additive to the toner and the elastic blade, and the stability of the tribo is spoiled by the deterioration of the elastic blade with time and the deterioration or the like of the toner due to the stress by the toner supplying and collecting roller. As a result, the developing characteristic is changed, and this has led to a great problem regarding durability.
Further, the magnetic carrier is not used and therefore the developing efficiency of the toner has been very bad, and the deterioration of image such as an edge enhancement has been liable to occur.
So, in order to solve the above-noted problems, as shown in FIG. 12 of the accompanying drawings, there has been proposed a developing method of taking out only a nonmagnetic toner component from a dual-component developer contained in a developer container 2, and conveying it to a developer carrying member and using it for development (see, for example, Japanese Patent Publication No. H06-19638).
In this developing device, a developer carrying member 8 is provided with a developing sleeve 8a and a magnet roll 8b provided in the interior of the developing sleeve 8a, and an elastic blade 82 is provided so as to abut against the developing sleeve 8a. Also, a nonmagnetic mono-component developer and a slight amount of magnetic particles 83 are contained in a developer container 2. When in this construction, the developing sleeve 8a is rotated in a direction indicated by the arrow E, the magnetic particles 83 in the developer container 2 are blocked by the elastic blade 82, and only the nonmagnetic mono-component developer is carried on the surface of the developing sleeve and is conveyed to a developing area opposed to a photosensitive drum 10.
This method is a developing method which can satisfy the stability of durability resulting from the use of the mono-component developer and a higher quality of image resulting from the use of the mono-component developer.
However, an improvement in low developing efficiency resulting from the use of the mono-component developer is not achieved.